[Tests] set $_ before sourcing nvm.sh in fast tests

nvm.sh uses `NVM_SCRIPT_SOURCE="$_"` to detect its source location.
Adding `: nvm.sh` before each source line ensures `$_` is set correctly, preventing breakage when the previous command (e.g., `set -ex`) overwrites it.

test/fast/Running 'nvm unload' should unset all function and variables

@@ -12,6 +12,7 @@ typeset -f | awk '/ \(\) $/ && !/^main / {print $1}' > "${BEFORE}"
 env | grep -v PATH= | grep -v IFS= | grep -v NVM_ | grep -v TRAVIS_ | sort >> "${BEFORE}"
 
 set +e # TODO: fix
+: nvm.sh
 \. ../../nvm.sh
 set -e